Aerodynamic window

ABSTRACT

A gas dynamic laser emits an output beam from a low pressure region downstream of a nozzle section through which gases are expanded which contain the constituents necessary to obtain a lasing action. A gas dynamic laser employs the principles of kinetic relaxation of excited states of specific gas species to effect a population inversion of the excited states to obtain lasing action. An oscillator directs a low beam into said low pressure lasing region, and through an optic arrangement the beam is amplified and directed out of an opening in the gas dynamic laser. Under normal circumstances the provision of such an opening would incur a flow of air from the ambient pressure to the low pressure within the laser cavity. The opening is connected to an aerodynamic window. The window has an unobstructed passage leading from the low pressure region of the gas dynamic laser to its exterior in which pressures are set up permitting passage of the laser beam, yet preventing or minimizing the low of exterior air into the cavity. The pressures are set up by an intersecting passageway having a pump supplying high pressure fluid and a nozzle construction which provides the proper predetermined pressures.

United States Patent I 1 3,654,569 Hausmann [45] Apr. 4, 1972 [5AERODYNAMIC WINDOW .[72] Inventor: George F. Hausmann, Glastonbury,Conn. [57] ABSTRACT A gas dynamic laser emits an output beam from a lowpressure [73] Asslgnee: United Aircraft Corpomfion East Ham regiondownstream of a nozzle section through which gases ford Conn areexpanded which contain the constituents necessary to ob- [22] Fil d; D23, 1968 tain a lasing action. A gas dynamic laser employs theprinciples of kinetic relaxation of excited states of specific gas spe-[2 l App]. No.: 786,485 cies to effect a population inversion of theexcited states to obtain lasing action. An oscillator directs a low beaminto said 52 us. Cl 331/945, 350/319 P lasing and s 0Ptic arrangemem 511 int. Cl. ..l-l0ls 3/05, HOls 3/08 the m is amplified and direckedinfile 58 Field of Search .350/319; 330/43; 331/945, dynamlc laser;Under "mmal clfcumstanFes P 331/945 such an opening would incur a flowof air from the ambient pressure to the low pressure within the lasercavity. The open- [56] References Cited ing is connected to anaerodynamic window. The window has an unobstructed passage leading fromthe low pressure region OTHE PU A S of the gas dynamic laser to itsexterior in which pressures are set up permitting passage of the laserbeam, yet preventing or Hurle Ehjctromc poFfulanon lnvers1n by Fluld'minimizing the low of exterior air into the cavity. The pres- MechamcalTechniques. 8 Physics of Fluids l60l 07-(1966). sures are set up b an ii g passageway having a pump supplying high pressure fluid and a nozzleconstruction which Primary ExammerBenlamm provides the properpredetermined pressures. Assistant ExaminerN. Moskowitz Attorney-Jack N.McCarthy 10 Claims, 3 Drawing Figures do 6/1 [A 746 AERODYNAMIC WINDOWCROSS-REFERENCES TO RELATED APPLICATIONS Application Ser. No. 731,658,filed May 23, 1968 is for an aerodynamic window having a differentarrangement for providing the proper predetermined pressures.

BACKGROUND OF THE INVENTION This invention relates to aerodynamicwindows and particularly for gas dynamic lasers. In lasers of low power,windows with physical walls made of materials which transmit the laserwavelength have been used, but subject window is for use when the laserbeam will disintegrate physical window materials.

SUMMARY OF THE INVENTION A primary object of this invention is toprovide a window which would permit passage of a laser beam with nophysical obstructions, yet prevent or minimize fiow through said win--dow between the two areas of varying pressures.

In accordance with the present invention, flow is compressed to providean oblique shock wave to prevent the fiow of a fluid through apassageway.

Further, a closed-loop system can be used to reduce the pressurerequirements of a pump.

BRIEF DESCRIPTION OF THE DRAWINGS =sion waves.

FIG. 3 is a view showing a plurality of windows between the interior ofthe cavity and the exterior thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT A gas dynamic laser 1 comprisesa nozzle 3, a laser cavity and a diffuser 7. The laser beam formed inthe cavity 5 is directed through an opening9 in the side of the lasercavity. An oscillator 4 directs a laser beam into said laser cavity 5and the beam is amplified between the mirror 6 and 8 to produce theoutput beam X. A passage II extends away from and around the opening 9through which the output beam passes from the cavity.

A pump 12 supplies high pressure air, or other gas, to a Laval nozzle 14which provides a uniform supersonic flow in a duct 15. Duct I5intersects the passage 11 for directing the flow of said nozzlethereacross. A duct 17 intersects said passage 11 at a point directlyacross from the intersection of the duct 15 to receive the flowtherefrom. The duct 17 extends into a diffusing section 19, nozzlethroat 21 and an expansion section 23 where it is then connected to theinlet of the pump 12.

At point A the Mach number is supersonic and the static pressure P, isadjusted by the pump total pressure and the area ratio of the Lavalnozzle 14 to be exactly equal to the static pressure P in the lasercavity. A wedge, or isentropic, surface 25 is provided to establish anoblique shock wave, or series of shock waves B, which causes an increasein the static pressure of the fiow and the turning of the flow to theangle d). The configuration of the compression surface 25 is designedsuch that the static pressure downstream of the shock wave or waves P isexactly equal to the atmospheric pressure P It is, thus, apparent thatwith P equal to P there is no flow across streamline C, and since P isequal to P, that there is no flow across streamline D.

A laser beam can be passed through this opening and there will be aminimum of flow in the atmosphere into the laser cavity 5. Thepositioning of the aerodynamic window is such that the intercept ofshock wave B is downstream of corner 27. The flow having a supersonicMach number in duct 17 may be exhausted directly to the atmosphere at alow supersonic Mach number. However, an optimization of pump pressurerequirements may be obtained by diffusing the flow at 19 which convertsthe kinetic energy of the flow to a static pressure which will be abovethe ambient value P,,. This flow is then ducted to the inlet of pump 12at a pressure P which is higher than atmospheric pressure P andtherefore provides a reduction in pump pressure ratio to establish thedesired operating characteristics.

Conventional compressible flow calculations indicate that a pressureratio of 10:! between the atmosphere and the laser cavity may besustained for a Mach number, M equal to 3.5 with a wedge compressionangle of 35. Assuming a total pressure recovery of between the flow induct 17 and the inlet to the pump 12, a pump pressure ratio on the orderof 3:1 or 4:1 will provide the desired static pressure ratio between theatmosphere and the laser cavity. Under normal conditions the pump may beoperated at constant RPM to effect the desired performance with nofurther control requirements. However, for sustained operation of theaerodynamic window, a heat exchanger 40 or other heat rejection devicewould be required to dissipate the adiabatic temperature rise of thepump.

A modified version of the aerodynamic window is shown in FIG. 2. In thiscase, the compression surface 25a downstream of noule 14 is designed tobe curved to provide a series of weak compression waves Bwhich'intercept at a point which is downstream of corner 27. The totalpressure drop across this series of weak compression waves is less thanthat across the single oblique shock formed by the wedge 25 in FIG. 1,and thereby reduces the pump pressure requirement. The flow in duct 17ais either exhausted to the atmosphere or is compressed through asupersonic diffuser 19a and a subsonic diffuser 21a prior torecirculation to the pump.

FIG. 3 shows a modification wherein a plurality of windows are usedbetween the interior of the cavity and the exterior thereof. This typeof arrangement is useful when the dif ferential between P,;, and P isvery great or when pump pressure and flow requirements must beminimized. The pressures and duct geometry within each individual windoware adjusted to provide a gradual pressure rise across the total windowsystem.

The window in this modification is different than shown in FIG. 1 andFIG. 2 and involves a configuration in which both compression wavesformed at compression surface 42 are used in conjunction with expansionwaves formed at expansion surface 47 to provide a greater staticpressure ratio across the aerodynamic window than could be obtained onlyby the use of a compression surface.

It is to be understood that the invention is not limited to the specificdescription above or specific figures shown, but may be used in otherways without departure from its spirit as defined by the followingclaims.

I claim:

1. In combination, a gas laser comprising a main duct, means providing agas containing constituents necessary to obtain a lasing action,expansion means in said duct, a lasing region of low pressure downstreamof said expansion means, reflector means in said main duct forming alaser output beam, an outlet in said main duct forming an exit for thereflected output laser beam, an outlet duct connected to said main ductand extending away therefrom around said outlet in said main duct, anauxiliary duct connected to said outlet duct and extending transverselythereacross, means providing supersonic flow through said auxiliary ductto form by compression waves a predetermined pressure difference acrosssaid exit duct, said auxiliary duct being formed at one part on one sideof said outlet duct as a Laval nozzle with its open exhaust end beingconnected to the outlet duct by a short duct section having one sideextending in a direction parallel to the center line of the Laval nonlewhile the opposite side is formed having an angle with respect to thecenter line of the Laval nozzle, said auxiliary duct being formed on theother side of said outlet duct as a passage extending at an angle tosaid outlet duct which is a continuation of the angle set forth above.

2. A device for providing an aerodynamic window between two regions atdifferent pressures including in combination, a passageway having aninlet and an outlet, said inlet being connected to one region and saidoutlet being connected to another region, a first transverse opening insaid passageway, a Laval nozzle, conduit means connecting the exit ofsaid nozzle to said first transverse opening, a compression wave formingsurface on one wall of said conduit means upstream of said firsttransverse opening, a second transverse opening being positioned in saidpassageway across from the first transverse opening, a collection ductextending away from said second transverse opening, a pump connected tothe inlet of said nozzle, a shock wave created by flow over saidcompression wave forming surface extends through the first transverseopening and intersects the opposite edge of the second transverseopening.

3. A device as set forth in claim 2, wherein the compression waveforming surface is a wedge.

4. A device as set forth in claim 2, wherein the compression waveforming surface is a curved surface.

5. A device as set forth in claim 2, wherein the wall opposite thecompression wave forming surface is formed as an expansion wave formingsurface.

6. A device as set forth in claim 5, wherein the compression waveforming surface and expansion wave forming surface extend from the exitof said nozzle at a substantially constant spacing to the firsttransverse opening in the passageway.

7. A device as set forth in claim 2, wherein the collection duct extendsat an angle away from the passageway.

8. A device as set forth in claim 7, wherein said angle is approximatelythat of the wedge surface.

9. A device as set forth in claim 2, wherein a subsonic diffuser islocated downstream of said collection duct, said diffuser having aninlet and outlet, said collection duct being connected to the inlet ofsaid diffuser.

10. A device as set forth inclaim 9, wherein the exit of the subsonicdiffuser is connected to the inlet of the pump, pressure ratio acrossshock wave and absolute pressure on vacuum side adjusted by pumppressure, nozzle area ratio, and angle of compression wave formingsurface.

1. In combination, a gas laser comprising a main duct, means providing agas containing constituents necessary to obtain a lasing action,expansion means in said duct, a lasing region of low pressure downstreamof said expansion means, reflector means in said main duct forming alaser output beam, an outlet in said main duct forming an exit for thereflected output laser beam, an outlet duct connected to said main ductand extending away therefrom around said outlet in said main duct, anauxiliary duct connected to said outlet duct and extending transverselythereacross, means providing supersonic flow through said auxiliary ductto form by compression waves a predetermined pressure difference acrosssaid exit duct, said auxiliary duct being formed at one part on one sideof said outlet duct as a Laval nozzle with its open exhaust end beingconnected to the outlet duct by a short duct section having one sideextending in a direction parallel to the center line of the Laval nozzlewhile the opposite side is formed having an angle with respect to thecenter line of the Laval nozzle, said auxiliary duct being formed on theother side of said outlet duct as a passage extending at an angle tosaid outlet duct which is a continuation of the angle set forth above.2. A device for providing an aerodynamic window between two regions atdifferent pressures including in combination, a passageway having aninlet and an outlet, said inlet being connected to one region and saidoutlet being connected to another region, a first transverse opening insaid passageway, a Laval nozzle, conduit means connecting the exit ofsaid nozzle to said first transverse opening, a compression wave formingsurface on one wall of said conduit means upstream of said firsttransverse opening, a second transverse opening being positioned in saidpassageway across from the first transverse opening, a collection ductextending away from said second transverse opening, a pump connected tothe inlet of said nozzle, a shock wave created by flow over saidcompression wave forming surface extends through the first transverseopening and intersects the opposite edge of the second transverseopening.
 3. A device as set forth in claim 2, wherein the compressionwave forming surface is a wedge.
 4. A device as set forth in claim 2,wherein the compression wave forming surface is a curved surface.
 5. Adevice as set forth in claim 2, wherein the wall opposite thecompression wave forming surface is formed as an expansion wave formingsurface.
 6. A device as set forth in claim 5, wherein the compressionwave forming surface and expansion wave forming surface extend from theexit of said nozzle at a substantially constant spacing to the firsttransverse opening in the passageway.
 7. A device as set forth in claim2, wherein the collection duct extends at an angle away from thepassageway.
 8. A device as set forth in claim 7, wherein said angle isapproximately that of the wedge surface.
 9. A device as set forth inclaim 2, wherein a subsonic diffuser is located downstream of saidcollection duct, said diffuser having an inlet and outlet, saidcollection duct being connected to the inlet of said diffuser.
 10. Adevice as set forth in claim 9, wherein the exit of the subsonicdiffuser is connected to the inlet of the pump, pressure ratio acrossshock wave and absolute pressure on vacuum side adjusted by pumppressure, nozzle area ratio, and angle of compression wave formingsurface.